1. Field of the Invention
The invention relates to lithium/metal sulfide energy storage batteries and particularly to novel positive electrodes for such batteries.
2. Description of the Prior Art
Development of energy storage batteries for load-leveling in electrical utilities during periods of peak power demand and as power sources for electric vehicles has been emphasized in recent years due to the increasing requirement for compact, pollution-free electrical power sources.
A major result of this effort has been the production of the lithium/sulfur battery, which generally comprises a lithium negative electrode, a molten salt electrolyte, and a positive electrode comprising sulfur and a current collector material. During discharge of such a battery, lithium is oxidized to lithium ions at the negative electrode by the reaction Li.fwdarw.Li.sup.+ +e.sup.-, which migrate through the electrolyte and react with the sulfur electrode to form lithium sulfide (Li.sub.2 S) by the reaction 2Li.sup.+ + S+2e.sup.- .fwdarw. Li.sub.2 S. The electrical energy generated by this reaction is removed from the battery through terminals provided therein. The battery may be recharged by supplying current thereto from an external source in a reverse direction, causing the lithium ions to migrate back to the negative electrode, where metallic lithium is formed by electron addition.
During the development of these batteries, it was found that sulfur used alone in the positive electrode tended to escape from the electrode both through vaporization and through the solubility of certain sulfur-bearing species in the molten salt electrolyte. Although these effects can be suppressed by additives such as arsenic or selenium, which lower the activity of the sulfur, it is questionable whether they are sufficiently effective to permit long battery lifetimes.
One of the most significant developments relating to lithium/sulfide power sources has been the discovery that selected metal sulfides are highly efficient as active materials in the positive electrodes of lithium/sulfur batteries. Metal sulfides were considered as alternatives to sulfur on the premise that the activity of sulfur in these compounds would be low enough to eliminate sulfur losses by vaporization or solubility, but not so low as to unduly decrease the battery voltage. ("Development of High-Specific-Energy Batteries for Electric Vehicles, " Progress Report for the Period August 1973- January 1974, Argonne National Laboratory, ANL-8058). From a practical standpoint, the metal sulfides of iron, cobalt, nickel, copper and mixtures thereof have the desirable characteristics of low cost, abundance, and lack of toxicity.